Since Tang and Vanslyke made the first multi-layer organic light-emitting diode by vacuum deposition of organic thin films at room temperature (Appl. Phys. Lett. 51, 913 (1987)), OLEDs as well as PLEDs have received considerable attention due to their potential applications in flat-panel display. As a result, more and more new materials and processing technologies have been developed to improve the performance of the devices. The main attention was paid to developing large-area flat panel displays with high contrast, high brightness, long-term stability, and low production cost.
To obtain high-performance light emitting devices with low carrier injection barriers, high electroluminescence (EL) efficiency and long lifetime, materials design and device configurations are two important factors. It is desirable that the materials possess the following properties: good carrier transport properties, high photoluminescence (PL) quantum yield, and suitable ionization potential (IP) and/or electron affinity (EA). Thus, the synthesis of highly fluorescent and stable materials that can be utilized in organic and polymer LEDs is one of the most challenging tasks in this field.
To all these aspects, electrodes are regarded as a most important component for the performance of the devices. In a basic organic and polymer LED structure, transparent conductive oxide (TCO) layer is used as the anode. A widely used TCO is tin-doped indium oxide (ITO) due to its high conductivity, work function and transparency over visible spectral range. Although ITO is probably the most successful TCO, indium is a relatively scarce element in the earth's crust. The estimated reserves are only 2600 metric tones and the current rate of extraction is approximately 230 metric tones annually. The cost for ITO production is therefore high. The other drawback of ITO films is the low chemical stability in a reduced ambient. The indium in the ITO layer can diffuse into the organic materials, leading to a degradation of the LED device performance.
In order to achieve the best device performance and to reduce the cost of device production, it is necessary to seek new electrode materials. In the present invention the midfrequency magnetron sputter deposited aluminum-doped zinc oxide (ZnO:Al) films are used as anode material for preparing LED devices. These films deposited at a temperature lower than 200 C. with a high deposition rate of about 10 nm/s exhibit low resistivity of 300 μΩcm. Furthermore, they are more stable in reducing ambient than ITO material.